"Because night-shift work has become very common in developed countries, future studies should assess the relationship of light exposure to the risk of other cancers and consider the risks in men," said team leader Dr Eva Schernhammer in the today's issue of Journal of the National Cancer Institute.

About 4% of adults work rotating night shifts in the United States, and similar numbers are common in industrialised countries. Shift work disrupts normal melatonin production and increases levels of other hormones such as oestrogen.

Women's cancers are often linked with oestrogen, but Dr Eva Schernhammer and her team suspect melatonin may play a more important role.

"While this finding needs to be replicated in future studies, the data is beginning to show that it may be melatonin, not estrogen, that is influencing cancer risk," she said. "If melatonin's anti-cancer properties are the source of our observed effects, this research opens a whole new arena of potential associations between exposure to light and a variety of cancers."

The researchers studied 78,586 women taking part in the Nurses' Health Study, first established in 1976. They found that nurses who worked night shifts at least three times a month for 15 years or more had a 35% greater risk of colon or rectal cancer.

Melatonin is produced at night, and regular exposure to sunlight affects the production cycle, which peaks in the middle of the night. Artificial light suppresses melatonin production.

"Melatonin has well established anti-carcinogenic properties, and a link between exposure at night and cancer risk through the melatonin pathway could offer one plausible explanation for the increased risk we observed," the researchers wrote.

They noted, however, that other factors - as yet unrecognised - could be missing, and urged further study.